Conventionally, machines driven by rotating a rotation shaft have been used, such as bicycles, automobiles, electric fans, belt conveyors, escalators and kneading machines. For example, a user drives a bicycle to run on the ground by turning a pedal which is a rotational driving source to generate propulsive power by transmitting a rotational driving force therefrom to the front and rear wheels through a chain. The bicycles as used in this manner include the so-called assist bicycle equipped with an electric motor as a rotational driving source and a battery to reduce the efforts required for turning a pedal when starting or climbing up a slope. The pedaling force exerted on this assist bicycle by human power is detected by a detection unit (torque sensor or the like), which outputs a signal indicative of the pedaling force to a control device which drives the electric motor in accordance with the pedaling force (for example, refer to the Patent Document 1).
On the other hand, another example of the machines which are rotationally driven is an electric fan equipped with a blade assembly which is provided with a plurality of blades and connected to the rotation shaft of an electric motor which is a power source. This electric fan can generate an air flow by transmitting the driving force of the electric motor to the vanes and rotating the blade assembly with the driving force (for example, refer to Patent Document 2).